This invention relates to a process for separating effectively unreacted monomers or solvents from synthetic rubber latex or synthetic resin latex by contacting the latex with an inert gas such as air, nitrogen or helium, etc. in an wetted-wall tower.
Two different methods have generally been used, heretofore, in order to separate monomers from latices. That is to say, a first method is the so-called steam distillation in which steam is blown into a vessel containing latices. This method has, however, a disadvantage such as the latex is degraded as a result of losing its stability and of the forming of coagulating matters owing to a long exposure of the latex to a high temperature. Moreover, it has another defect that the decrease of the solid content of the latex gives rise to trouble, in general, because of the condensation of steam during the processing.
A second method is to subject latices to vacuum distillation. However, this method also has disadvantages in that it takes a long period of time to treat the latex owing to its vigorous foaming in apparatuses, and that it is required to use an apparatus with a much larger capacity in comparison with the amount to be treated owing to deal with a foaming latex. Some antifoaming agents may be added to the latex in order to remove these defects, but may cause the contamination of the latex in this case. It is another defect of this method that the latex is degraded owing to the formation of drying latex film or of coagulating matters resulting from water being eliminated from the surface of the latex during vacuum distillation.
The two methods described above can be operated by means of both a batch process and a continuous process from viewpoint of the type of apparatus. In case that latices are treated with apparatuses known heretofore, an apparatus for a batch process has disadvantages that the latex is degraded as a result of its long exposure to a higher temperature resulting in lower productivity and efficiency of the apparatus. And apparatuses used for a continuous processing have some defects as follows: since an apparatus for increasing its efficiency gives mechanical shear stress to latices, there forms coagulating matter; apparatus with large capacity is difficult to make; the price of apparatus is high; the shape and structure of apparatus is complex, and the cleaning of apparatus inevitable in case of treating latices is hard to do.
In addition to the above methods, as a method for treating latex, for example, there is a method disclosed in U.S. Pat. No. 3,003,930. According to this method, the latex is treated by feeding it to a stripping tower having a number of plates with special structure disposed inside in alternately oblique directions. However, the plate is merely inclined at an angle of 1 to 45 at most, and the latex therefore does not fall down vertically. Moreover, in this method, there is nothing described with regard to bringing the latex into contact with inert gas. Furthermore, in the portion from line 72 in column 2 to line 6 in column 3 of the patent, it is clearly descriped that plates having a special structure are used whereby the contact of the vapor (which is not an inert gas but a liquid vapor contained in the latex) flowing upward from the lower part of the tower with the latex can be prevented. That is to say, the contact of the vapor and the latex is rather positively evaded.
In such a method, any liquid incorporated in the latex can not be fully stripped. In fact, in the U.S. Patent, it is described that the latex once treated is heated and recycled again, and that a number of towers are operated under vacuum.
In U.S. Pat. No. 2,514,207, there is disclosed a method in which water and unreacted monomers are removed from the latex by batch operation. In this method, the latex is taken in a reaction vessel with an agitator, and water and unreacted monomers are recovered from the latex by means of stripping, heating and treating under vacuum of its liquid. This method is one of the conventional processes for treating latices heretofore known. However, it makes use of outernal force such as stirring operation, mechanical shear stress acts upon the latex, and the stability of latex is largely destroyed to form coagulating matter. In addition, the operation under vacuum often causes foaming in the vessel, which brings about reduced stripping effects and the difficulty of the operation itself.
In the portion from line 58 in column 2 to line 9 in column 3 of the patent, it is described that the heated latex to be recycled is supplied onto the slinger ring and that a thin film of the latex is formed on the wall surface of the stripper. However, the film, as is clearly described in column 3, line 3, fills the role of washing the walls and preventing the accumulation of any solid matter on them (accumulation seems to be formed by adhesion onto the wall surface of the latex splashed by agitation).
As is mentioned above, a variety of methods to eliminate solvents used and unreacted monomers in the treatment of latices have been widely studied by those skilled in the art. Latices, however, are different from a solution and contain water, solid particles, emulsifiers, solvents and monomers, etc. in definite balanced conditions, therefore when treating latices, there arise such problems as the decrease of the stability of latices, the formation of coagulating material and foaming, etc. Accordingly, an excellent method to solve these problems has been earnestly desired heretofore.
This invention is converned with providing a superior process to these problems.